A comparison of neutron diffraction and hole-drilling residual strain measurements in thermally sprayed coatings

Abstract

A critical factor dictating the durability of WC metal matrix composite coatings is the residual stress profile in the coating/substrate system. Modelling and development of future generation coatings require experimental evaluation of the residual stress field. The experimental evaluation of the stress field in the coating/substrate system can be sensitive to the measurement technique. The aim of this investigation was to study this dependency in thermally sprayed multilayer WC–NiCrBSi coatings in the as-sprayed and HIPed conditions. Incremental hole drilling was applied to semi-destructively evaluate the residual stress field in the coating/substrate system. Results were compared to previously reported findings of residual stress using neutron diffraction. The results of this investigation indicate that in a complex multilayer system the two measurement methods provide complementary information about the overall residual stress field in the coating/substrate materials. Residual stress values evaluated by both measurement techniques were similar in the top 300-μm-thick WC-10wt.% NiCrBSi coating layer, whereas significant differences were recorded in the intermediate 100-μm-thick WC-40wt.% NiCrBSi coating layer. The difference in the measured values for the two measurement techniques is attributed to the nature of the stress measured i.e. macro-stress for hole-drilling as opposed to micro-stress in the WC coating phase for the neutron diffraction technique. Additional factors such as the assumptions of defect-free coating microstructure perfectly bonded to the underlying coating layer and the lower content of WC-phase in the intermediate and diffusion coating layers, along with the formation of secondary phase particles in the HIPed coating microstructure, contributed to the differences between the measured residual stress values for the two techniques.